WDB ENCOUNTERED SEVERE TURBULENCE. PASSENGER AND CREW WERE INJURED.

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THE FLT ENCOUNTERED SEVERE TURB AT FL410 FOR 5-10 SECS. PRIOR TO THE TURB THE FLT WAS IN SMOOTH AIR AND WAS FLYING IN AND OUT OF A CIRRUS LAYER. THE RADAR WAS ON. THE SCALE HAD BEEN REDUCED FROM THE 80 MI RANGE TO THE 40 MI RANGE WITH 1 DEG DOWN TILT. FOR THE 80 MI PRIOR TO ENCOUNTERING THE TURB. THE ONLY RETURN ON THE RADAR WAS SMALL GREEN PRECIPITATION RETURN WHICH WOULD PASS 15 MI W OF OUR INTENDED TRACK. JUST AS THE FLT EMERGED FROM A CIRRUS LAYER; THE VAGUE OUTLINE OF WHAT APPEARED TO BE THE TOP OF A C/B APPEARED DIRECTLY AHEAD OF THE ACFT AND 200-300' ABOVE OUR ALT. THE SUN WAS BELOW THE HORIZON. FOR ALL PRACTICAL PURPOSES IT WAS NIGHT WITH LIMITED FORWARD FLT VIS. SIMULTANEOUSLY THE ACFT WAS TURNED TO THE RIGHT TO AVOID THE C/B; THE SEATBELT SIGN WAS TURNED ON AND ATC WAS ADVISED OF OUR TURN. ALMOST INSTANTANEOUSLY AS WE STARTED THE TURN; THE AIRPLANE ENTERED PART OF THE CLOUD. THE FLT ENCOUNTERED SEVERE TURB FOR 5-10 SECS. THE INJURIES IN THE CABIN TO THE PAX AND F/A'S WERE TO THE EXTENT THAT THE FLT WAS DIVERTED TO ATC FOR MEDICAL ASSISTANCE. IN RETROSPECT WITH THE LIMITED FORWARD FLT VIS AND ABSENCE OF A RADAR RETURN; I DO NOT BELIEVE THAT THE TURB AND SUBSEQUENT F/A INJURIES COULD HAVE BEEN AVOIDED. A LARGE PERCENTAGE OF THE PAX INJURIES COULD PROBABLY HAVE BEEN PREVENTED HAD THEY HEEDED THE ANNOUNCEMENT WHICH OUR F/A'S MAKE AFTER THE SEATBELT SIGN IS TURNED OFF. THIS ANNOUNCEMENT CAUTIONS THE PAX TO KEEP THEIR SEATBELTS FASTENED EVEN WHEN THE SIGN IS OFF. THE F/O AND I COULD HAVE BEEN MORE RESPONSIVE TO THE PROBS IN THE CABIN HAD A THIRD CREW MEMBER BEEN PRESENT. EVEN WITH THE AUTOMATION OF THE WDB; FLYING THE AIRPLANE; HANDLING THE COMS WITH ATC AND YOUR COMPANY AND CONFERRING WITH THE F/A IN CHARGE DURING A PROB JUST ABOUT PUSHES A 2-PLT CREW PAST THE POINT WHERE THEY MIGHT BE ABLE TO ADEQUATELY HANDLE THE AIRPLANE.